1. Field of the Invention
The present invention relates to a fuel-cell power-generation system which includes a plurality of fuel-cell stacks and provides a large quantity of electric power.
2. Description of the Prior Art
As is well known, fuel cells are a continuously operating type electrochemical power-generation system which includes a stack comprising many unit cells superimposed one on another, the stack having a fuel electrode and an oxidant electrode, and which operates by supplying hydrogen gas to the fuel electrode side and oxygen (air) to the oxidant electrode side of the stack to allow electrochemical reaction between the hydrogen and oxygen to take place, and converting energy from such a reaction directly and efficiently into direct-current electrical energy.
In the fuel-cell of this type, because of limitations posed thereon in the power capability per stack, i.e., the size of a unit cell, and number of superimposed cells, in view of restrictions from production technologies, and those from transportable size; it is difficult to provide stacks having power capabilities of actually required levels. In particular, it is currently a major problem to increase the power capability of fuel-cell power-generation plant. However, in order to obtain a fuel-cell power-generation plant having a high power capability, it is necessary to combine a plurality of stacks of one and the same specification with each other so that the stacks can be connected electrically in parallel connection or in series-parallel connection because there is a limitation on the power capability per stack of the fuel-cell power-generation system.
In the case where the fuel-cell system with a plurality of stacks in parallel connection is to be operated as described above, some modifications have been made to the single-stack fuel-cell construction. First, a fuel feed line arranged between the stack and a fuel reformer, which converts natural gas or the like as a raw fuel into a hydrogen-rich fuel usable in the fuel-cell system, and an air feed line arranged between the air blower and the stack have in their mid portion a plurality of sublines or branch lines branched therefrom the number of which corresponds to the number of the stacks used, the branch lines being connected to the corresponding stacks, respectively. Then, based on a standard utilization factor of fed gas which has been set up to such a level that the system can operate as a power generation and electricity supply system in the best power-generation efficiency. (e.g., hydrogen utilization factor: 75%, oxygen utilization factor: 50%), the gas is distributed to each stack through each branch line with controlling the amount thereof being controlled collectively.
Generally, even among fuel-cell stacks designed and produced in accordance with the same specification, the power characteristics (current-voltage or I-V characteristics) of the unit cells would be different to some extent from individual to individual as shown in FIG. 1, due to fluctuation or difference in the quality of parts such as electrodes constituting the unit cell, the amount of electrolyte impregnated in each unit cell, and the conditions under which the unit cell is operated. To note, FIG. 1 shows respective I-V characteristics for two fuel-cell stacks A and B.
Assuming the case where a power-generation system is used which comprises two fuel-cell stacks A and B with different cell power output characteristics, electrically connected to each other in parallel connection, and is operated with feeding the reaction gas such that its amount necessary for the overall system calculated from the electric load and standard gas utilization factor is divided into two parts, one for the stack A and the other for the stack B. In this case, as will be apparent from FIG. 1, some difference arises between currents I.sub.A and I.sub.B (current for the overall system being I.sub.A +I.sub.B) allotted to the stacks A and B, respectively, for establishing an output voltage V.sub.O. More specifically, the electric load is allotted to the fuel-cell stacks A and B unevenly or based on other than fifty-fifty basis, but one of the two stacks, e.g., the stack A, having a low cell characteristics has a smaller ratio of allotment of the load and on the contrary the other, stack B, having a high cell characteristics has a greater ratio of allotment of the load.
As the amounts of hydrogen and oxygen consumed by the fuel-cell power-generation system is proportional to the amount of current with a coefficient of the Faraday constant, the uneven allotment of the electric load between the stacks A and B results in that the hydrogen and oxygen would not be consumed in accordance with the predetermined utilization factors of the gases in the stack A, with a relatively small ratio of allotment of electric load and less current and unused gases are discharged to the outside of the system, whereas in the stack B with a relatively great ratio of allotment of electric load the amounts of reaction gases are insufficient as compared with the current allotted to the stack B, thus increasing the gas utilization factor to above the standard value and causing the state of so-called gas-deficit operation, i.e., the system is operated in short of fuel gas. Once the fuel-cell power-generation system is run in the state of gas-deficit operation, the occurrence cell reaction is localized at the gas inlet side of each unit cell, and as a result localized overheating occurs in the unit cell. This localized overheating not only decreases the output characteristics of the system but also injures the cell to shorten the cell-life, and thus in the worst case it is impossible to operate the system.
As general characteristics of fuel cells, it is known that continuous operation of fuel cells at high loads results in increased amount of electrolytes scattered, which increases the rate at which the characteristics of the fuel cell are deteriorated. Accordingly, in order to prolong the cell-life for the entire power-generation system, it is necessary to operate the system at ratios of allotment of load for respective fuel-cell stacks arranged in parallel connection as even as possible among the stacks.